1. Field of the Invention
The invention pertains generally to flux valve compass systems for navigable craft and more particularly relates to the compensation of undesirable changes or errors in the signal characteristics in flux valve data repeater systems, including errors due to variation in the horizontal component of the earth's field, index angle errors, and cardinal and intercardinal heading errors.
2. Description of the Prior Art
When navigating at high latitudes while utilizing flux valve magnetic compass systems, difficulty is often experienced because of the decreasing strength of the horizontal component of the earth's magnetic field as latitude increases. A flux valve type of magnetic compass normally senses only the horizontal component of the earth's field, so that, at high latitudes, the strength of the sensed horizontal component is proportionally lessened, and the compass system experiences decreasing sensitivity, resulting in heading information of diminished accuracy.
Prior art systems have sought to solve this compensation problem of providing an input to the compass data repeater substantially independent of variations in the strength of the horizontal component of the earth's field by controlling the gains of amplifiers or the effective values of impedances in the separate channels of the data transmitter system in a relatively complex manner, but generally in inverse relation to the signal strength as measured at the flux valve itself. Examples of prior arrangements are described by D. A. Espen in the U.S. Pat. No. 3,548,284 for "Synchro Data Transmission Apparatus Having Discrete Gain Changing to Compensate for Undesirable Signal Gradient Variations", issued Dec. 15, 1970, and by J. R. Erspamer and G. W. Snyder in the U.S. Pat. No. 3,646,537 for an "Automatic Gain Control for an Electromechanical Transducer", issued Feb. 29, 1972, both patents being assigned to the Sperry Rand Corporation. While these concepts have been useful in providing adequate magnetic field compensation in many circumstances, the compensating signals compensate only for variation in the horizontal magnetic field components, and generally do not additionally correct fully for gain changes due to component variations or due to temperature or power supply voltage drifts or to component aging. Further, the characteristics of the individual gain control elements of the individual channels of the data system may vary without proper corrective relative adjustments, resulting in the generation of two-cycle transmission errors by the gain control stages.
The improved system disclosed by J. R. Erspamer and G. W. Snyder in the U.S. Pat. 3,784,753, issued Jan. 8, 1974 and entitled: "Multiplexed Gain Control for a Synchro Data Transmission System", sought more fully to overcome these prior art defects by a relatively complex and expensive correction circuit. Though it generally overcame such defects, it was found that some undesirable two cycle error could be generated in its complex automatic gain control stage, and that a simple way was needed for identically chaning the gains of channels of the data transmission system, but retaining the advantages of the concept of U.S. Pat. No. 3,784,753.
Prior art systems have additionally sought to provide correction for the index angle error in compass data transmission systems by use of networks including precision differential synchros or ganged dual potentiometers which much track each other with high precision if they are not themselves to introduce errors. According to the present invention, the expense of obtaining such selected precision potentiometers or synchros is desirably eliminated. Cardinal heading error was similarly corrected in prior compass data transmission systems by using precision ganged dual potentiometers of similar quality and it is found increasingly desirable to eliminate costly components and to substitute simpler and less expensive networks permitting a single adjustment control for each of these corrections and, at the same time, retaining high precision.